HYDROLOGICAL CHARACTERIZATION OF A SULPHIDE WASTE ROCK DUMP

Abstract

Acid rock drainage (ARD) from sulphide bearing waste rock dumps poses a serious threat to the environment and has become problematic to the mining industry. Water that is discharged from sulphide waste rock dumps has the potential to be low in pH, thus having the ability to transport heavy metals. The acid water and the heavy metals in solution became toxic to the environment. Acid rock drainage from sulphide bearing waste rock dumps is the most serious environmental liability in the mining industry; believed to be $3.2 billion for 750 million tonnes of waste rock in Canada alone (Feasby et al., 1997). The understanding of the characteristics and quantity of water flow through waste rock has become fundamental. A complete hydrologic characterization was performed for the sulphide waste rock dump at Equity Silver Mine Ltd. near Houston, Be (575 km north northwest of Vancouver, Canada). The characterization of the hydrologic system entailed the investigation of five elements: geologic structure, topography, surface hydrology, groundwater and water chemistry. The hydrologic budget was determined for the waste rock dump. The components are as follows: precipitation, runoff, sublimation, mass transfer, evapotranspiration, changes in storage, infiltration and groundwater. Precipitation was measured with an on site weather station. The runoff was measured for the 1998 freshet with a series of weirs and culverts that were instrumented to measure runoffwater. The remaining surface components were determined by the SoilCover (1997) model, a one dimensional finite difference heat and mass transfer program. The groundwater component was investigated using a numerical model, FEMW A TER (ECGL, 1998), which can solve three dimensional saturated or unsaturated groundwater flow regime systems.

All of the surface hydrological components are required in order to equalize the surface water balance for the waste rock dump. The components of the surface hydrological budget during the one year study period over the area of the waste rock dump are as follows: precipitation of 642 mm, 94 rnm (15 %) runoff, 327 mm (51 %) evapotranspiration, 27 rnm (4 %) infiltration, 97 mm (15 %) sublimation and 97 rnm (15 %) mass transfer. The cover system lost 9 rnm of water during the one year study period; thus the net surface infiltration was 36 rnm (6 %). The water balance relationship for the acid rock drainage collection ditch that surrounds the waste rock dump was evaluated. The contributions to the ditch are: runoff, infiltration, groundwater discharge and changes in storage. The water balance for the drainage ditch showed that the acid rock drainage flow reporting to the ditch is equivalent to 318 rnm of water per year over the area of the waste rock dump. The components of this total flow are estimated to be 36 mm (11 %) infiltration, 27 mm (9 %) runoff, 252 mm (79 %) groundwater discharge and 3 mm (1 %) due to changes in storage within the waste rock.